The present invention relates to a method of processing a selected surface in a semiconductor process chamber by contacting the selected surface with a reactant that reacts preferentially with the selected surface relative to another surface when the selected surface is at a different temperature from the other surface. The invention is particularly suited for use in rapid thermal processing (RTP) systems. More particularly, the invention relates to the use of nitrogen trifluoride (“NF3”) gas to clean selected surfaces in RTP process chambers, including in Rapid Thermal Chemical Vapor Deposition (RTCVD) process chambers.
During the course of semiconductor device fabrication, a wafer undergoes many steps of processing. Some steps involve heating the wafer hundreds of degrees Celsius above ambient temperature. Unnecessary heating may lead to defective devices being formed on the wafer. One reason for this is that heat causes substances, such as implanted ions, to migrate outside their intended location on the wafer. Migration and other things affected by heating are compounded by the length of heating time and by the number of heat steps. To avoid the problems associated with wafer heating, it is advantageous to use processes that minimize the time that a wafer is kept at an elevated temperature. This is the principle of conserving the thermal budget of the wafer.
In a conventional process chamber using a tube furnace, for example, it might take several minutes to raise the temperature of the wafer to a desired level. Because the walls and other surfaces in such process chambers are directly heated along with the wafer, such systems can be referred to as “hot wall” systems. The heated walls and other surfaces in the chamber also increase the wafer's cooling time. Thus, in hot wall process chambers a wafer may be subject to prolonged heating and cooling.
In contrast, RTP process chambers use a radiant energy source, such as high intensity lamps, to rapidly heat a wafer to a desired temperature in a matter of seconds. The energy source may raise the wafer's surface temperature by 75-125° C. per second. Other surfaces in the chamber are not generally heated by the radiant energy—for example quartz liners and windows in the chamber may be transmissive of the radiant energy frequencies or the energy may not be directed onto the surfaces. Because the radiant energy source does not directly heat these surfaces, RTP systems may be referred to as “cold wall” systems.
Because the rapid heating and cooling in RTP systems conserve a wafer's thermal budget, it was expected that RTP systems would be widely adopted for every heat-based process. However, RTP systems have certain disadvantages that have not been adequately addressed. One significant disadvantage is that radiant energy may not be absorbed uniformly by a wafer. This results in temperature variations in and across the wafer. Temperature variations lead to non-uniform process results. RTP systems therefore require sensitive systems for monitoring and controlling the temperature of the wafer. One useful system uses a parallel plate with a reflective coating that reflects energy to the backside of the wafer. An optic fiber collects and transmits the wafer's backside emissivity to a detector that translates the frequencies of energy to a temperature reading. Unfortunately, a significant disadvantage of this temperature monitoring system is its vulnerability to certain cleaning processes carried out in an RTP process chamber.
This problem and certain others addressed by the present invention are illustrated in the following example: the deposition of polysilicon on a wafer in an RTCVD process chamber. Deposition of polysilicon is a known step in the fabrication of certain semiconductor devices. One notable process using polysilicon is the formation of hemispherical grained silicon (“HSG”). HSG formations enhance the storage capacitance in storage devices such as Dynamic Random Access Memory Arrays (“DRAMS”). Poor, irregular HSG formations result in unclean RTCVD process chambers. (Methods for forming HSG are described, for example, in U.S. Pat. Nos. 5,634,974 and 5,759,262 which are hereby incorporated by reference as if set forth in their entirety.)
An undesired side effect from polysilicon deposition is that the polysilicon is deposited not only on the wafer but also on other surfaces in the process chamber. For example, in RTCVD process chambers, it accumulates on, among other things, the top quartz window and quartz liner of the chamber. A build-up of polysilicon impedes the transmission of radiant energy through the window onto the wafer. It may also lead to high particle counts in the chamber, lowering production yields. Therefore, periodic cleaning of the process chamber is necessary.
NF3 is a powerful etchant, effectively etching polysilicon and certain other substances used in semiconductor processing. Use of NF3 for in-situ cleaning of a non-RTP semiconductor process chambers is described in U.S. Pat. No. 5,797,195, to Hulling et al, entitled “Nitrogen Trifluoride Thermal Cleaning Apparatus and Process.” The '195 patent describes a “hot wall” system for cleaning semiconductor fabrication equipment, including quartzware parts. NF3 gas is heated from approximately 100° C. to 650° C. by the existing heat source for the process chamber. At the same time, the heat source also heats the other surfaces in the process chamber, which surfaces the NF3 gas is free to contact. The '195 patent does not teach or suggest that the described cleaning system is suitable for use in “cold wall”, RTP systems, including RTCVD systems.
It would be desirable to use NF3 gas to clean RTP process chambers, particularly those used to deposit polysilicon in HSG production. However, NF3's high reactivity has so far limited its usefulness in this regard. NF3 cleaning, until the present invention, has not proved suitable for use in cold wall systems, such as RTCVD systems, because certain components in RTCVD chambers may be damaged by the high activity of NF3 at higher temperatures.
As the temperature of NF3 increases, its reactivity increases, converting to reactive species in the form of ionic fluorine and/or free fluorine. The temperatures that occur in RTCVD processing can exceed 750° C. At such temperatures, NF3 is so corrosive that it attacks stainless steel, quartz, and silicon surfaces, damaging critical components found in RTCVD process chambers. As mentioned, temperature-monitoring systems based on wafer emissivity are particularly vulnerable to damage. NF3's action on such unintended targets may also create unacceptably high particle counts in the process chamber, lowering production yields.
If the NF3 cleaning is performed at lower temperatures, the etching rate is too low to effectively clean the system. This is because most of the hardware inside the process chamber, including the top quartz window, is heated by heat transmitted from the wafer, not by light used to heat the wafer. Therefore, if the temperature of the wafer is low, the top quartz window is even cooler. This results in unacceptably low etching rates.
For the foregoing reasons, a method is needed that allows a reactant introduced into an RTP process chamber to act preferentially on certain surfaces. Among other things, such an improved method would allow strong etchants, such as NF3, to clean polysilicon deposits from process chambers without damaging sensitive components in the chamber.